Composite material, tubing made from the material, and methods for making the material and tubing

ABSTRACT

A strip of composite metal laminate material embodying a thin inner layer of stainless steel sandwiched between and metallurgically bonded to two relatively thicker outer layers of low carbon steel is subjected to a brief, high temperature heat treatment followed by a relatively much longer heat treatment at much lower temperature for substantially eliminating the yield point in the laminate and for maximizing formability of the laminate while permitting some reduction in the corrosion resistance properties of the stainless steel layer of the laminate. The composite strip material is then readily formed into two, concentrically disposed convolutions of the strip material while the laminate material is in this highly formable condition. The convolutions of the strip material are then brazed together to provide a double-walled tubing, the brazing procedure being regulated to effect selected heat treatment of the formed composite material for substantially restoring the corrosion resistance properties of the stainless steel layer of the laminate within the finished tubing.

.United States Patent [191 Dromsky [451 Nov. 27, 1973 [75] Inventor:John A. Dromsky, North Attleboro,

Mass.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Mar. 16, 1972 [21] Appl. No.: 235,297

Related US. Application Data [62] Division of Ser. No. 100,007, Dec. 21,1970, Pat. No.

3,712,317 1/1973 Hayashi et a1. 29/l96.l

Primary Examiner-W. W. Stallard Attorney1-larold Levine et a1.

[57] ABSTRACT A strip of composite metal laminate material embodying athin inner layer of stainless steel sandwiched between andmetallurgically bonded to two relatively thicker outer layers of lowcarbon steel is subjected to a brief, high temperature heat treatmentfollowed by a relatively much longer heat treatment at much lowertemperature for substantially eliminating the yield point in thelaminate and for maximizing formability 3696-499- of the laminate whilepermitting some reduction in the corrosion resistance properties of thestainless steel [52 US. Cl. 148/12, 148/123 layer of h iaminata Thecomposite strip material is [51] Int. Cl. CZld 1/78, C2ld 9/08 thenreadily formed into two, concentrically disposed [58] Field of Search148/123, 12, 34; convolutions of the Strip materiai while the laminate29/196-l material is in this highly formable condition. The convolutionsof the strip material are then brazed together [56] References C'ted toprovide a double-walled tubing, the brazing proce- UNITED STATES PATENTSdure being regulated to effect selected heat treatment 2,653,117 9/1953Keene 29/196.1 0f the formed Composite material for Substantially 3,392,437 7/1968 29/196.1 storing the corrosion resistance properties ofthe stain- 3,393,445 7/1968 Ulam 29/196.1 less steel layer of thelaminate within the finished tub- 3,499,803 3/1970 Henrickson et al29/196.1 i 3,537,828 1l/l970 Henrickson et a1 29/196.l 3,648,353 3/1972Anderson 29/1961 1 Claim, 5 Drawing Figures I BOA/0 D 5544 p44 75 FORMEfi/IZE ANA/144 ANA/54L I ROLL PATENTEUNUYZT 1915 3.775 194 0 a I fST/PA/A/ (2 62, 0/1/04 r/o/v) COMPOSITE MATERIAL, TUBING MADE FROM THEMATERIAL, AND METHODS FOR MAKING TIIE MATERIAL AND TUBING This is adivision, of US. Pat. application Ser. No. 100,007, filed Dec. 21, 1970now US. Pat. No. 3,696,499.

Conventional automotive brake tubing and the like is manufactured byforming low carbon steel strip material into concentrically disposedconvolutions of the material and by brazing the convolutions together toform a double-walled tubing construction. This conventional tubing ismanufactured at relatively low cost because the industry has developed awell standardized process and equipment for manufacturing tubing of thisconstruction in very large quantities. More recently, it has beenproposed that such tubing be formed from a strip of composite metallaminate material embodying a thin inner layer of stainless steelsandwiched between and metallurgically bonded to two relatively thickerouter layers of low carbon steel. It is found that this compositematerial is readily formed in the manner of low carbon steel to providethe desired double-walled tubing configuration, is readily brazed in themanner of the low carbon steel material previously used in such tubing,and provides the resultant tubing with significantly increasedresistance to corrosion, particularly including pitting types ofcorrosion. However, it is desirable to provide such composite laminatematerial in an economical way to permit even greater ease in formingmaterial into the desired tubing configuration.

It is an object of this invention to provide a novel and improvedcomposite metal laminate material for use in making brazed,double-walled tubing; to provide such a material which is readily formedinto the desired double-walled tubing configuration; to provide such atubing material which is readily brazed using conventional brazingequipment to form the desired double-walled tubing; to providedouble-walled tubing made from such composite material; to provide suchtubing which displays excellent resistance to corrosion particularlyincluding pitting types of corrosion; to provide such tubing which is ofeconomical construction, and to provide novel and improved methods formaking said composite metal laminate material and for making saiddouble-walled tubing.

Briefly described, the composite laminate material of this inventioncomprises at least one, thin inner layer of stainless steel which issandwiched between and metallurgically bonded to two relatively thickerouter layers of low carbon steel. This composite material is initiallyformed in any conventional manner by rolLsqueezing the metal layers ofthe laminate together, preferably at room temperature, with substantialreduction in the thickness of the metal" layers to form the desiredmetallurgical bonds between the metal layers. In accordance with thisinvention, the composite metal material is then heated to a relativelyhigh temperature on the order of 1,850 F. or more for a brief period oftime for substantially annealing the stainless steel layer of thelaminate material. The composite material is then heated to a relativelylower temperature on the order of 1,100 F. or less for a substantiallylonger period of time. It is found that this latter heat treatment stepsubstantially softens the low carbon steel layers of the laminate andmodifies the properties of the laminate so that the laminate is highlyformable and displays substantially no yield point during'subsequentdeformation of the laminate. It is also found that this latter heattreatment tends to cause some undesirable reduction in the corrosionresistance properties of thestainless steel layer of the laminate.

The resulting composite material is then formed into two concentricallydisposed convolutions of the strip material to form a tubingconfiguration in a conventional manner, these convolutions of the stripmaterial then being heated and brazed together for securing theconvolutions together to form the desired doublewalled tubing. Theheating of the composite material is performed for accomplishing brazingof the tubing material is regulated for substantially restoring thecorrosion resistance properties of the stainless steel layer of thelaminate material.

In this way, it is found that the desired'layers of metal aremetallurgically bonded together in a laminate material in a secure andeconomical way. The described heat treatment of the composite materialpermits the composite material to be very easily formed into the desiredtubing configuration and modifies the properties of the materialsembodied in the composite so that the composite material displayssubstantially no yield point during deformation of the material into thedesired tubing configuration. Further, the deformed laminate material isreadily brazed in a conventional manner for securing convolutions in thelaminate material together to form the desired double-walled tubingconstruction, the heat treatment which is employed in brazing alsoserving to modify the properties of the laminate material forsubstantially restoring the initial high resistance to corrosion of thestainless steel layer of the laminate.

Other objects, advantages and details of the composite material, tubingand methods of this invention appear in the following more detaileddescription of preferred embodiments of the invention, the detaileddescription referring to the drawings in which:

FIG. 1 is a block diagram illustrating process steps in the manufactureof composite laminate materials and tubings in accordance with thisinvention;

FIG. 2 is a section view to enlarged scale along the transverse axis ofa strip of composite material provided by this invention;

FIG. 3 is a section view to enlarged scale along the transverse axis ofthe tubing provided by this invention;

FIG. 4 is a section view similar to FIG. 2 illustrating a step in theprocess of forming the tubing of FIG. 3 from the composite material ofFIG. 2; and

FIG. 5 is a graph illustrating characteristics of the composite materialof FIG. 2.

Referring to the drawings, the block diagram of FIG. I illustrates thegeneral steps contemplated for making a double-walled tubing inaccordance with this invention. That is, as is indicated at 10 in FIG.ll, the first step in forming the desired tubing construction calls forrollbonding of a plurality of metal layers to form a composite metallaminate 12 as shown in FIG. 2, the metal laminate embodying a thininner layer 14 of stainless steel which is sandwiched between tworelatively thicker outer layers 16 of low carbon steel and which ismetallurgically bonded to the low carbon steel layers substantiallythroughout the interfaces '18 between the metal layers. Roll-bondingprocesses for making such composite metal laminate materials are wellknown and will be understood that, in such roll-bonding processes,strips of annealed steel materials or the like having sur- Mati ee Vfaces which have been cleaned for removing bonddeterrent substancestherefrom are initially brought into interfacial contact with eachother. The contacting metal strips are then squeezed together between apair of pressure rolls with substantial reduction in the thicknesses ofthe strip materials, thereby to metallurgically bond the strips to eachother to form a composite metal laminate in which the strip materialsform respective layers of the laminate. Usually the resulting bondedcomposite material is heated for sintering and strengthening the bondsbetween the metal layers of the composite and for intermediatelyannealing or stress relieving the materials of the composite and thecomposite is then rolled again for further reducing the compositematerial to a desired final gauge or thickness. Such rollbondingprocesses are illustrated, for example, in U.S. Pat. Nos. 2,691,815 and2,753,623. As such rollbonding processes are well known and as variousconventional roll-bonding processes can be employed in making thelaminate material 12 within the scope of this invention, formation ofthe laminate 12 by rollbonding is not further described herein and itwill be understood that the laminate 12 embodies layer materials asabove described, that the layers of the laminate material aremetallurgically bonded together substantially throughout theirinterfacially contacting surfaces, and that materials of the compositelaminate have been work-hardened to at least some extent by thereduction in thickness of the laminate layer materials which hasoccurred during roll-bonding of the laminate or during subsequentrolling of the laminate to a desired final gauge or thickness.

In accordance with this invention, the composite metal laminate 12embodies various conventional steel materials. For example, the thininner layer 14 of stainless steel material in the laminate preferablycomprises an austenitic stainless steel such as Type 304 Stainless Steelhaving the composition, by weight, as set forth in Table I but is alsoformed of various other conventional stainless steel materials such asthe additional materials set forth in Table I within the scope of thisinvention. Similarly, the layers 16 of the laminate 12 are preferablyformed of low carbon steels such as Type 1008, Low Carbon Steel havingthe composition, by weight, as set forth in Table II but is also formedof various steel materials such as the additional materials set forth inTable II within the scope of this invention. Various types of low carbonsteels including rimmed, capped and aluminum killed low carbon steelsare suited for such use. The stainless steel layer 14 of the laminatematerial 12 desirably comprises from 2 to 25 percent, and preferablyabout percent, of the total thickness of the composite metal laminatematerial.

In accordance with this invention, the composite metal laminate material12 as initially fonned by rollbonding, or as additionally rolled tofinal gauge or thickness after bonding, is heated to a relatively hightemperature for a relatively brief period of time for substantiallyannealing the thin inner layer 14 of stainless steel material in thelaminate as is diagrammatically illustrated at 20 in FIG. 1. In thisregard, it is noted that the laminate material as bonded and rolled hasusually under gone about to 70 percent reduction in thickness since theprevious intermediate annealing or heat treatment of the material of thecomposite. The laminate material is therefore subjected to this firstheat treatment step for annealing the stainless steel material to removethe work-hardening of the stainless steel which had occurred duringroll-bonding of the laminate or during rolling reduction of the bondedlaminate to a desired final gauge. Preferably, the laminate material isheated in a reducing atmosphere at a temperature in the range from about1,850 to about 2,050 F. for a period from about one-half to 2 minutesfor accomplishing strip annealing of the stainless steel layer of thelaminate, the strip of laminate material being fed through anyconventional strip annealing furnace or the like for accomplishing thisannealing step. For example, in preferred embodiments of this invention,the laminate material 12 is advanced through a reducing atmosphere in aconventional strip annealing furnace at a temperature in the range from1,850 to 2,000 F., and preferably at a temperature of 1,950 F., thestrip having a residence time of about 1 minute in the furnace. Theheated laminate material is then preferably cooled to about 150 F.within about one minute by passing the material through a coolingreducing atmosphere, the material then being coiled and permitted tocool to room temperature. In this regard, the strip of laminate materialsent through the strip annealing furnace is provided with very clean,dry and reduced surfaces which would tend to stick together quiteeasily. Accordingly, coiling of the strip of laminate material 12 aftersaid strip annealing is preferably regulated in any conventional mannerto minimize tension in the coil of the laminate material to avoidsticking together of the outer surfaces of the laminate duringsubsequent batch heat treatment of the coil of laminate material.

In accordance with this invention, as is indicated at 22 in FIG. 1, thecomposite metal laminate material 12 is then subjected to additionalheat treatment at a relatively much lower temperature for a relativelymuch longer period of time. For example, the metal laminate 12 afterhaving been subjected to a strip annealing type of heat treatment in themanner above described is TABLE I Silicon Phosphorous Sulfur Manganese(max.) Chromium Nickel (1110K) (mfllL) Iron 5. -7. 50 1.00 16.00-18.003. 50-5. 50 0. 06 0.030 Balance 1 7. 50-10. 00 1.00 17. 00-19. 00 4.00-0. 00 0.06 0.030 D0.

1 2.00 1.00 16.00-18.00 6.00-8.00 0. 045 0.030 D0. 1 2.00 1.00 17.00-19. 00 8. 00-10. 00 0. 045 0. 030 D0. 1 2. 00 1.00 18.00-20. 00 8.00-12. 00 0. 045 0. 030 D0. 2 2.00 1. 00 17. 00-10. 00 J. 00-12. 00 0.045 0.030 D0.

1 Type 201 and 202 stainless steels incorporate 0.25 percent, by weight,(1110):.) oiiiitrogen mid Type 321 Stainless steel additionallyincorporates approximately 0.40 percent, by weight, of titanium.

TABLE II 1 Maximum.

22 leaves the low carbon steel layers of the laminate material in anannealed, highly formable condition. As will be understood, this latterheat treatment to the laminate materials is preferably performed in anyconventional manner in a bell or batch annealing furnace or the like.

In forming tubing from the laminate 12 which has been prepared in themanner described above, the heat-treated composite metal laminate ispreferably provided with relatively thin outer surface layers of copperor other brazing material as is diagrammatically indicated at 24 in FIG.1 and as it is illustrated at 26 in FIG. 4. Preferably, for example, theheat-treated laminate 12 is passed through a conventional electrolyticcopper plating step for depositing thin copper layers 26 having athickness on the order of 0.00015 inches on the outer surfaces of thelaminate, the lateral edges of the strip of laminate material then beingtrimmed in any conventional manner for providing unplated edges 12.1 onthe laminate strip as illustrated in. FIG. 4. The plated strip oflaminate material is then formed into a double-walled tubingconfiguration as is diagrammatically indicated at 28 in FIG. 1 and isbrazed as is diagrammatically indicated at 30 in FIG. 1, thereby to formthe tubing 32 as illustrated in FIG. 3. That is, the plated laminatestrip material 12 is deformed around the longitudinal axis of the strip12 to form an inner .-convolution 34 of the strip material which isconnected to an outer convolution 36 of the strip material by anintegral, crossover portion 38 of the strip material so that the innerand outer convolutions of the strip material are concentrically disposedwith respect to each other. The formed and plated strip material 12 isthen heated to a temperature in the range from about 1,980 F. to about2,070 F. for a period of time from about k to 2 minutes for melting thelayers 26 of copper brazing material to braze the inner and outerconvolutions 34 and 36 of the strip material together and to braze thelateral edges 12.1 of the strip material to respective opposite sides ofthe crossover portion 38 of the strip material, thereby to form tubing32 as shown in FIG. 3. As will be understood, this copper brazingmaterial is indicated at 26 in FIG. 3 where it is shown that the layersof copper brazing material on the laminate have flowed together in thetubing 32. It should be noted that the plating, forming and brazing ofthe strip material 12 to form that tubing 32 as above described comprises substantially the same process steps as are conventionallyemployed in forming double-walled automotive brake tubing and the likefrom monolithic, low carbon steel strip material. Accordingly thelaminate is readily formed and brazed to provide the tubing 32 using themanufacturing equipment conventionally employed in forming brazed,double-walled automotive,

brake tubing and the like. As these processes are well known, theplating, forming and brazing techniques as described above need not befully described herein and it will be understood that these processsteps serve to form the plated laminate material into the tubingconfiguration as shown in FIG. 3 and serve to braze portions of thestrip material to each other to form the tubing 32, the laminatematerial 12 being subjected to the noted temperatures for the abovenoted periods of time during such brazing.

When the composite metal laminate material 12 is formed and processed inthe manner above-described to provide the double-walled tubing 32, anumber of process and product advantages are obtained. First,

ally corresponds to a process step conventionally ,used

in treating steel materials so that each of the process steps is adaptedto be performed on conventional processing equipment. The processingrequired in accordance with this invention is therefore economical toperform. Further, the laminate material provided by the notedcombination of process steps displays an excellent degree of formabilityand is readily formed into the desired tubing using conventionaltube-forming equipment. The laminate material also displayssubstantially no yield point during the deformation required for formingthe tubing so that the tube-forming deformation of the laminate occursuniformly and is easily controlled and so that the deformed tubingmaterial is free of the surface appearance defects usually associatedwith deforming material such as annealed low carbon steels whichcustomarily display very pronounced yield points. Further, the tubingprovided by this invention displays excellent resistance to corrosion,particularly including pitting types of corrosion.

In this regard, the noted advantages of this invention are believed toresult from the interaction of several factors. For example, it has beennoted above that, after forming of the laminate 12 by roll-bonding andafter rolling reduction of the laminate to a desired final thickness,the stainless steel and low carbon steel materials of the laminate havebeen substantially workhardened so that the formability of the laminateis relatively low. If the laminate material is to be utilizedeconomically in producing brazed, double-wall tubing as described above,the formability of the laminate must be increased to make the materialmore compatible with the equipment conventionally used for making suchdouble-walled tubing. In the first, strip annealing typeof heattreatment in the process of this invention as indicated at 20 in FIG. l,the stainless steel layer 141 of the laminate 12 is substantially fullyannealed for enhancing the formability of the stainless steel layer ofthe laminate. Then, during the second, longer and lower temperature heattreatment of the laminate'indicated at 22 in FIG. 1, interstitialelements such as carbon and nitrogen in the low carbon steel layers 16of the laminate are removed from solid solution and are precipitated inthe low carbon steel, thereby substantially softening and enhancing theformability of the low carbon steel layers of the laminate. In thisregard, while it is understood that the long, slow heat treatment of thelaminate tends to compromise and reduce the corrosion resistanceproperties of the stainless steel. layer of the laminate by permittingoccurrence of some intergranular carbide precipitation in the stainlesssteel, this reduction in the corrosion resistance of the stainless steelis tolerated in the laminate in the interest of optimizing theformability of the low carbon steel layers of the laminate. The longslow heat treatment of the laminate is preferably performed at atemperature above l,O F. to achieve the desired increase in formabilityof the low carbon steel materials while minimizing intergranular carbideprecipitation in the stainless steel layer of the laminate. In this way,the laminate material is adapted to be readily formed into the desireddoublewalled tubing configuration illustrated in FIG. 3 usingconventional tube-fon'ning equipment. Then, subsequently, during brazingof the deformed laminate material to form the tubing 32, the laminatematerial in the tubing is subjected to brazing temperatures in the rangefrom l,980 to about 2,070 F. for a period of from about to 2 minutes,this final heat treatment of the laminate resulting in resolution of theprecipitated carbides in the stainless steel layer of the laminate forsubstantially restoring the original corrosion-resistance properties ofthe stainless steel layer of the laminate.

With regard to the fact that the heat-treated laminate material 12 ofthis invention displays substantially no yield point during deformationof the laminate to form the tubing 32, it is noted that the presence ofinterstitial carbon contributes significantly to the occurrence of theyield point phenomenon in low carbon steel materials. The heat-treatmentsteps of the process of this invention, in precipitating suchinterstitial carbon from the low carbon steel materials in the laminate12, thus significantly affects the yield point characteristics of thelaminate. Further, during the long, low temperature heat treatment ofthe laminate 12 noted above, significant diffusion of carbon from thelow carbon steel materials of the laminate into the stainless steelmaterial of the laminate is believed to occur, this diffusion of carbonout of the low carbon steel further affecting the yield pointcharacteristics of the laminate. Finally, it is noted that the stainlesssteel material of the laminate 12 does not ordinarily display a yieldpoint so that the presence of the stainless steel layer 14 in thelaminate is believed to dampen any tendency of the low carbon steelmaterials of the laminate to display the yield point phenomenon. Thesethree factors are thus believed to cooperate so that the heat-treatedlaminate material 12 does not display a yield point during formation ofthe laminate into the tubing 32.

In a preferred embodiment of this invention, for example, a cleanedstrip of Type 304 stainless steel in annealed condition having a widthof 12.50 inches and a thickness of about 0.015 inches is advancedbetween a pair of cleaned strips of Type 1008 aluminum killed, lowcarbon steel having a similar width and a thickness of about 0.067inches, the strips being advanced together into the nip between a pairof squeezing rolls wherein the strips are squeezed together withsubstantial reduction in the thicknesses of the strips formetallurgically bonding the strips together in conventional manner. Theresulting, bonded composite metal laminate material is then subjected toan intermediate anneal in conventional manner and is rolled to a finalgauge or thickness of 0.014 inches in which the central stainless steellayer of the laminate comprises approximately percent of the totalthickness of the laminate. At this point, the composite material hasbeen reduced about 40 to 70 percent in thickness after the notedintermediate anneal and has relatively low formability. In accordancewith this invention, the composite material is then advanced through areducing atmosphere in a conventional strip annealing furnace at atemperature of 1,950 F., the strip having a residence time ofapproximately one minute in the furnace. At this point the compositematerial displays an upper yield point of 46,000 psi., a tensilestrength of 59,000 psi. and an elongation (in 2 inches) of about 35percent. The composite-laminate material is then heated in a slightlyreducing atmosphere comprising a mixture of cracked city gas andnitrogen in a-conventional bell furnace, the material being held at atemperature of 1,100 F. for a period of approximately 4 hours. At thispoint, the composite material displays no yield point but displays a 0.2percent offset yield strength of about 26,000 psi., a tensile strengthof about 56,000 psi. and

an elongation (in 2 inches) of about 40 percent. That is, thestress-strain curve for the composite metal laminate is as illustratedin FIG. 5. At this point, the stainless steel layer of the laminate isfound to have undergone some intergranular carbide precipitation anddisplays less than the optimum corrosion resistance of Type 304Stainless Steel. This composite material is then plated with copper oneach side thereof to a thickness of 0.00015 inches in conventionalmanner and is edge trimmed. The plated and trimmed material is thenformed in a conventional way to provide a doublewalled automotive braketubing configuration as illustrated in FIG. 3 having an outer diameterof about 0.187 inches. The tubing is then heated in a conventional tubefurnace so that the copper plating on the composite laminate is meltedfor brazing formed portions of the composite material together inconventional manner for securing the composite material in the notedtubing configuration. In this tube furnace, the formed compositematerial is heated to a temperature of 2,050 to 2,070 F. with aresidence time of about 1.125 minutes in the furnace. At this point, itis found that the initial corrosion resistance properties of thestainless steel layer of the laminate have been substantially restored.Where other stainless steel and low carbon steel materials as set forthin Tables I and 11 are embodied in similar laminates formed into similartubings using similar heat treatments, similar results are obtained.

It will be understood that although particular embodiments of thelaminate material, tubing and processes of this invention have beendescribed above by way of illustration, modifications and equivalents ofthe disclosed embodiments are within the scope of this invention. Forexample, some of the advantages of this invention are obtained where thestrip annealing type of heat treatment indicated at 20 in FIG. 1 iseliminated from the process of this invention. Similarly, although thetubing 32 is shown to be formed of a single strip of the laminatematerial 12, tubings formed from two or more strips of the laminatematerial 12 in conventional manner are also within the scope of thisinvention. Similarly, although the composite metal laminate material isshown to incorporate a single inner layer of stainless steel sandwichedbetween two relatively thicker outer layers of low carbon steel,advantages of this invention can also be obtained in a laminate materialformed of two relatively thin layers of stainless steel materialinterleaved with three relatively thicker layers of low carbon steelmaterial or in laminate materials having other generally similarlaminate layer constructions within the scope of this invention, thethree layer laminate described by way of illustrating this inventionrepresenting the preferred embodiment of the invention. It should beunderstood that although particular embodiments of the invention areillustrated, this invention includes modifications and equivalentsthereof falling within the scope of the appended claims.

I claim:

1. A method for making a formed element of a composite metal laminatematerial embodying at least one layer of stainless steel and at leastone layer of low carbon steel, said method comprising the steps ofheating said laminate material to a selected temperature for a selectedperiod of time for annealing said stainless steel layer of said laminatematerial, heating said laminate material to a relatively lowertemperature for a relaing said laminate material into a selected shape,and heating said laminate material to induce resolution of intergranularcarbide precipitates in said stainless steel material.

